1. Field of the Invention
The present invention is directed to a pressure pulse generator of the type having a pressure pulse source which creates acoustic pressure pulses in a liquid acoustic propagation medium by means of a concussively driveable membrane, also known as an electrodynamic pressure pulse generator.
2. Description of the Prior Art
Electrodynamic pressure pulse generators are used for a large variety of purposes. For example, such pressure pulse generators are used for medical purposes to non-invasively disintegrate calculi in the body of a patient, or to non-invasively treat pathological tissue conditions. For the first purpose, positive pressure pulses (greater than atmospheric pressure) are used, and in the latter application, negative pressure pulses (less than atmospheric pressure) are preferably used. Such pressure pulse generators, for example, may also be used in materials testing to charge material specimens with pressure pulses.
The pressure pulse generator is always acoustically coupled in a suitable manner to the subject to be acoustically irradiated, so that the pressure pulses generated in the acoustic propagation medium can be introduced into the subject with a minimum of reflections and energy loss. The pressure pulse generator and the subject to be acoustically irradiated must therefore be aligned relative to each other so that the region of the subject to be acoustically irradiated is located in the propagation path of the pressure pulses. If the pressure pulse generator is of the type which produces focused pressure pulses, it must also be assured that the region of the subject to be acoustically irradiated is located in the focal region of the focused pressure pulses.
A pressure pulse generator of this type is described in U.S. Pat. No. 4,674,505. This pressure pulse generator is a so-called electromagnetic shockwave generator which generates positive pressure pulses. This is accomplished by supplying high-voltage pulses to an electrically conductive coil arrangement, thereby causing the rapid build-up of a magnetic field. An electrically conductive membrane is disposed opposite the coil arrangement, and this magnetic field induces a current in the membrane in an opposite direction to the current flowing in the coil arrangement. The membrane current also is accompanied by a magnetic field, which is opposite in direction to the magnetic field associated with the coil arrangement. Repulsion forces are thereby rapidly produced, causing the membrane to be concussively moved rapidly away from the coil arrangement. The membrane interacts with an acoustic propagation medium to produce a pressure pulse therein, which gradually intensifies to form a shockwave along its propagation path.
A problem in pressure pulse generators of this type is that the membrane must be returned to its initial position after a pressure pulse has been generated. Only by doing so is it insured that the membrane will assume a defined position initial position before generating a further pressure pulse. It is important that the membrane assume such a defined initial position in order for successively generated shockwaves to coincide with sufficient precision with respect to their acoustic characteristics. A pressure pulse generator is disclosed by European Application 0 188 750, corresponding to U.S. Pat. No. 4,697,588 wherein return of the membrane to its initial position is accomplished by charging that side of the membrane facing away from the acoustic propagation medium with an under-pressure. Although the membrane is reliably returned to its initial position by this approach, a rather substantial design outlay is required and an under-pressure (suction) source must be provided.
A pressure pulse generator of the type described above is also disclosed in German OS 34 43 295, corresponding to U.S. Pat. No. 4,669,472, wherein the membrane is returned to its initial position by the acoustic propagation medium, which is maintained at a static pressure sufficient to accomplish this result. This approach has the disadvantage that the acoustic propagation medium adjacent the membrane cannot be conducted through a degasification means during operation of the pressure pulse generator, not can it be circulated through a cooling system in the manner disclosed by European Application 0 265 741, corresponding to U.S. Pat. No. 4,977,888. Degasification is desirable for removing gases dissolved in the acoustic propagation medium to prevent the formation of gas bubbles, which degrade the propagation of the pressure pulses. Cooling of the propagation medium is also desirable because a large amount of heat is dissipated during operation of the pressure pulse source, which must be eliminated in order to protect the pressure pulse source against premature failure due to elevated operating temperatures, primarily failure of the membrane which is subjected to high mechanical stresses.